1. Field of the Invention
The present invention relates to a crane in which a strut, to which a jib support pendant is connected, is installed at one or both of a leading end portion of a boom and a base end portion of a jib provided on a leading end side of the boom, so that the strut can be rotationally moved in a front-back direction.
2. Description of the Related Art
A luffing crane includes a crane main body including a lower traveling member and an upper slewing member, and a front attachment including a boom (main jib) with a foot thereof attached to the upper slewing member for derricking, a luffing jib (hereinafter simply referred to as a jib) with a foot thereof attached to a leading end portion (boom top) of the boom for rotational movement in an up-down direction, and a jib derricking apparatus.
Base ends of a front strut (also referred to as a front post) and a rear strut (also referred to as a rear post) are attached to the jib derricking apparatus near the foot of the jib or at the boom top and outside a direction in which the jib bends with respect to the boom so that the front strut is located closer to the jib than the rear strut and that the base end portions are rotationally movable in a front-back direction. For the operation and attitude of the struts, a direction in which the strut is proximate to the jib is hereinafter referred to as front, and a direction in which the strut is proximate to the boom is hereinafter referred to as back or rear.
A leading end portion of the front strut is joined to a leading end portion of the jib via a jib supporting leading end side pendant with a length dimension set to a certain value. A leading end portion of the rear strut is connected to a base end portion of the boom via a jib supporting base end side pendant with a length dimension set to a certain value.
Moreover, the leading end portion of the front strut and the leading end portion of the rear strut each include a plurality of sheaves. A jib hoist includes the sheaves of each strut and a wire rope paid out from a jib drum mounted on the crane main body and passed between the sheaves so as to form a plurality of strands.
For the luffing crane, the pieces into which the front attachment is disassembled are normally transported to the site, where the pieces are assembled into the front attachment, which is then attached to the crane main body.
In the assembly of the front attachment, the boom and the jib are sequentially connected to the crane main body on the ground so as to be linearly arranged. Subsequently, the jib derricking apparatus is assembled.
In the assembly of the jib derricking apparatus, first, the base end portions of the front strut and the rear strut are attached to the vicinity of the foot of the jib or to the boom top so as to be rotationally movable in the front-back direction. Then, over the front strut leaned forward along the jib, the rear strut is placed in an angular attitude in which an acute angle is subtended between the front strut and the rear strut. A wiring operation is then performed in which a jib-hoisting wire rope drawn out from the jib drum is passed around the sheaves at the leading end portion of the front strut and the sheaves at the leading end portion of the rear strut.
Then, the rear strut is rotationally moved backward and placed in a backward leaning attitude in which an acute angle is subtended between the rear strut and the boom.
Then, the leading end portion of the backward-leaning rear strut is connected to a second end portion of the base end side pendant with a first end portion thereof connected to the base end portion of the boom, the base end side pendant being disposed above the boom in a slackened manner.
Thereafter, the rear strut is rotationally moved forward to tense the base end side pendant.
After the base end side pendant is tensed, opposite end portions of the leading end side pendant are connected between the leading end portion of the front strut leaned forward along the jib and the leading end portion of the jib.
As means for rotationally moving the rear strut in the front-back direction, a configuration has been proposed in which a cylinder is interposed and attached between a longitudinally intermediate portion of the rear strut and a position on the boom close to the leading end thereof. Such a configuration allows the rear strut to rotationally move forward and backward in conjunction with an extending operation and a contracting operation, respectively, of the cylinder (see, for example, Japanese Patent Application Laid-open No. H8-48489).
However, since the base end side pendant attached between the base end portion of the boom and the leading end portion of the rear strut is set to a certain given length dimension, the tension of the base end side pendant increases rapidly with decreasing degree of slack of the base end side pendant during an operation of rotationally moving the rear strut in the backward leaning angular attitude forward to tense the slack base end side pendant. Thus, when the base end side pendant is tensed, a force needed to rotationally move the rear strut forward gradually increases.
The configuration disclosed in Japanese Patent Application Laid-open No. H8-48489 is such that the rear strut is rotationally moved forward by pushing the longitudinally intermediate portion of the rear strut forward from the rear side by the extending operation of the cylinder interposed between the rear strut and the boom. Thus, when the base end side pendant is tensed, in use of the cylinder the longitudinally intermediate portion of the rear strut needs to be pushed forward by a force in a pushing direction stronger than the tense of the base end side pendant, which acts on the leading end portion of the strut.
Thus, the rear strut is subjected to a heavy bending load by a force exerted on the longitudinally intermediate portion by the cylinder when the base end side pendant is tensed. This leads to the need for a bending strength sufficient to resist the bending load. Hence, disadvantageously, the components of the rear strut correspondingly need to be larger and heavier.
Furthermore, according to the conventional configuration, the cylinder is subjected to a load in a compressing direction as a force reactive to the strong force in the pushing direction, and thus needs to be strong enough to resist buckling.
Thus, an object of the present invention is to provide a crane which can eliminate the need to place a bending load on the longitudinally intermediate portion of the strut when the slack jib support pendant is tensed by rotationally moving the strut connected to the leading end portion of the pendant, which enables a reduction in the structural bending strength of the strut and thus in the weight of the strut, and which can prevent the load in the compressing direction from acting on a member connected to the strut to transmit, to the strut, a force applied by the direct acting apparatus to tense the pendant.